Bài giải:

${{a}^{2}}+{{b}^{2}}-{{a}^{2}}{{b}^{2}}+ab-a-b$

= $\left( {{a}^{2}}+ab \right)-\left( a+b \right)-\left( {{a}^{2}}{{b}^{2}}-{{b}^{2}} \right)$

= $a\left( a+b \right)-\left( a+b \right)-{{b}^{2}}\left( {{a}^{2}}-1 \right)$

= $\left( a-1 \right)\left( a+b \right)-{{b}^{2}}\left( a-1 \right)\left( a+1 \right)$

= $\left( a-1 \right)\left[ \left( a+b \right)-{{b}^{2}}\left( a+1 \right) \right]$

=$\left( a-1 \right)\left( a+b-a{{b}^{2}}-{{b}^{2}} \right)$

=$\left( a-1 \right)\left[ a\left( 1-{{b}^{2}} \right)+b\left( 1-b \right) \right]$

=$\left( a-1 \right)\left( 1-b \right)\left( a+ab+b \right)$

Đáp án đúng là A